fix: tighten dashboard vertical spacing

Reduce spacing now that top header is removed:
- Main content top padding: pt-6 → pt-4
- Header section: mb-8 → mb-6
- Chart section: mb-8 → mb-6
- Grid sections: mb-8 → mb-6
- Site name margin: mb-2 → mb-1
- Header inner gap: mb-4 → mb-3

components/dashboard/DashboardShell.tsx

@@ -38,7 +38,7 @@ export default function DashboardShell({
       />
       <div className="flex-1 flex flex-col min-w-0 overflow-hidden">
         <ContentHeader onMobileMenuOpen={openMobile} />
-        <main className="flex-1 overflow-y-auto pt-6">
+        <main className="flex-1 overflow-y-auto pt-4">
           {children}
         </main>
       </div>